Alkyl aromatic compounds have long been produced from hydrocarbon fractions relatively rich in such materials. Early sources were liquids from coking or other distillation of coals. More recently, these products have been derived from fractions obtained in refining of petroleum and other fossil hydrocarbons such as shales and bitumens. An important source in recent years has been the aromatic liquid naphthas resultant from severe thermal cracking of gases and naphthas to produce olefins. A major present source is reformed naphtha prepared by processing a petroleum naphtha over a catalyst having an alumina base with one or more platinum group metals dispersed thereon, alone or in admixture with other metals such as rhenium.
However derived, these aromatic rich streams have usually been distilled or otherwise separated (e.g. solvent extraction) to obtain the desired product components. It has also been proposed to concentrate the aromatics by hydrocracking. See Mason U.S. Pat. No. 3,037,930. The purpose of these prior practices and of the present invention are well typified by a product of present major importance and techniques for providing the same at requisite high levels of purity. Reference is made to para-xylene, now used in huge quantities for manufacture of terephthalic acid to be reacted with polyols such as ethylene glycol to make polyesters.
The major raw material for p-xylene manufacture is catalytic reformate prepared by mixing vapor of a petroleum naphtha with hydrogen and contacting the mixture with a strong hydrogenation/dehydrogenation catalyst such as platinum on a moderately acidic support such as halogen treated alumina at temperatures favoring dehydrogenation of naphthalenes to aromatics, e.g. upwards of 850.degree.F. A primary reaction is dehydrogenation of naphthenes (saturated ring compounds such as cyclohexane and alkyl substituted cyclohexanes) to the corresponding aromatic compounds. Further reactions include isomerization of substituted cyclopentanes to cyclohexanes, which are then dehydrogenated to aromatics, and dehydrocyclization of aliphatics to aromatics. Further concentration of aromatics is achieved, in very severe reforming, by hydrocracking of aliphatics to lower boiling compounds easily removed by distillation. The relative severity of reforming is conveniently measured by octane number of the reformed naphthas, a property roughly proportional to the extent of concentration of aromatics in the naphtha (by conversion of other compounds or cracking of other compounds to products lighter than naphtha).
To prepare chemical aromatics, a fraction of the reformate is prepared by distillation which contains six carbon atom and heavier (C.sub.6 +) compounds. That fraction is extracted with a solvent which is selective to either aromatics or aliphatics to separate the two types of compounds. This results in a mixture of aromatic compounds relatively free of aliphatics. Generally the fractionation preceding extraction is such that the fraction contains aromatics of six to eight carbon atoms, generally designated BTX for benzene, toluene, xylenes, although the fraction also contains ethyl benzene (EB).
Liquids from extremely severe thermal cracking, e.g. high temperature steam cracking of naphtha, are also rich in aromatics and may be used to prepare BTX in a manner analogous to that applied for reformate. Such liquids, sometimes called "pyrolysis gasoline" may be partially hydrogenated to convert diolefins or otherwise pretreated in the course of preparing BTX.
Concentrated aromatic fractions are also provided by severe cracking over such catalysts as ZSM-5 (Cattanach U.S. Pat. Nos. 3,756,942 and 3,760,024) and by conversion of methanol over ZSM-5.
From pure BTX, benzene and toluene are easily separated by distillation, leaving a C.sub.8 fraction containing the desired p-xylene. A portion of the EB can be separated as such from the other C.sub.8 aromatics, but the respective boiling points are such that substantially complete separation of EB requires "superfractionation" in elaborate, expensive distillation equipment requiring great operating expense. If EB is substantially completely removed, p-xylene may be recovered by fractional crystallization or selective sorption on solid porous sorbents. The remaining mixture of o-xylene and m-xylene is then subjected to isomerization and the isomerizate recycled to p-xylene separation with fresh charge. This constitutes a closed system herein called the "separation-isomerization loop" or simply the "loop". In some instances o-xylene is recovered by distillation and sold.
Processes are now available which will tolerate considerable amounts of EB in feed to the loop. This tolerance arises from use of an isomerization catalyst which will convert EB. "Octafining" is such a process now in wide use. It employs a catalyst of platinum on silica-alumina which concurrently isomerizes xylenes and converts EB in part to xylenes and in part to benzene and light products easily separated by distillation in the loop. Another proprietary process having similar effect is known as "Isomar". Certain crystalline aluminosilicate zeolites have been found to be effective for isomerization at specific conditions of xylenes which contain EB. These appear to act by disproportionation and dealkylation of EB to benzene and C.sub.9 + alkyl aromatics (e.g. methyl ethyl benzene or diethyl benzene) also easily separable by distillation. Those techniques are described in copending applications Ser. Nos. 397,039 now U.S. Pat. No. 3,856,872, 397,195 now U.S. Pat. No. 3,856,874, 397,194 now U.S. Pat. No. 3,856,873, and 397,038 now U.S. Pat. No. 3,856,871, all filed September 13, 1973. The zeolites so applied are typified by the highly versatile material designated zeolite ZSM- 5 as described and claimed in U.S. Pat. Nos. 3,702,886 and 3,790,471.
Zeolite ZSM-5 has also been described as extraordinarily effective in processing of aromatic-containing materials in the nature of light and full range reformates. See U.S. Pat. Nos. 3,767,568 and 3,729,409. In that context, ZSM-5 acts to crack straight chain and singly branched paraffins of low octane number and alkylate aromatic rings with the cracked fragments. Although there are indications that new aromatic rings are generated, the principal effect is increased octane number by increasing the weight percent of high octane aromatic compounds in light reformate by increasing molecular weight of benzene and other low boiling aromatics.
It is here appropriate to note that zeolite beta has been reported as a catalyst for conversion of C.sub.9 aromatics to C.sub.8 aromatics. See U.K. Specification 1,343,172. This and other descriptions of using crystalline zeolites for processing alkyl aromatics to prepare chemical products (as contrasted with treating reformates for motor fuel) generally employ a restricted aromatic mixture as feed to the zeolite catalyzed process, except for the four copending applications cited above. For example, xylene isomerization with zeolites is usually demonstrated with a single xylene or mixture of xylenes, free of EB. Zeolites have been shown to be effective catalysts for isomerization, transalkylation (including disproportionation), alkylation and dealkylation of benzene and alkyl benzenes.